HSDPA (High Speed Downlink Packet Access) for WCDMA (Wideband Code Division Multiple Access), a key new feature of Release 5 3GPP (Third Generation Partnership Program) technical specifications for UTRAN (Universal Mobile Telecommunications System Terrestrial Radio Access Network) of type FDD (frequency division duplex), makes possible increased packet data throughput—compared to what is possible using DCH (the dedicated channel), DSCH (the downlink shared channel) or FACH (the forward access channel)—by means of fast physical layer (L1) retransmission and transmission combining and also fast link adaptation, both controlled by the Node B/base transceiver station, not the radio network controller (RNC). The fast physical layer retransmission and transmission combining is part of a so-called HARQ (hybrid automatic repeat request) process, in which, in what is called incremental redundancy (IR), a wireless terminal stores received data packets in soft memory, and if decoding has failed, a retransmission is combined with the failed one before again attempting decoding. Besides soft combining related to retransmission in general, IR also includes soft combining where different versions of redundant encoded data are sent on each (re)transmission. (In basic and alternative so-called Chase combining mode, each retransmission contains an identical copy of a first transmission, which is also then “soft-combined” with earlier received symbols before attempting new decoding.)
With HSDPA, two features used generally in WCDMA—variable SF (spreading factor) and fast power control—are disabled and replaced by adaptive modulation and coding, multi-code operation, and HARQ. In addition, scheduling decisions (i.e. scheduling channel resources for downlinking packets) are made in the Node B (not the RNC), with a strategy of providing increased throughput by allowing most of the cell capacity to be allocated to a single user for a very short time, under some circumstances.
As shown in FIG. 1, in HSDPA packet data are downlinked using HS-DSCH (high-speed downlink shared channel) for the packet data itself and HS-SCCH (high-speed shared control channel) is used for the corresponding physical layer control information (code allocation and decoding information, and that information also supports soft combining in case of retransmission). HS-DPCCH (high-speed dedicated physical control channel) is used by the terminal to convey to the Node B ACK/NACK signalling (positive and negative ARQ acknowledgements), and feedback information on the quality of the downlink.
Communication over the HSDPA channels occurs as frames of subframes provided during TTIs (transmission time intervals), i.e. one subframe is one TTI (2 ms for HSDPA). A single subframe on HS-DPSCH may be used to convey a data packet. In order for a terminal to be able to decode a data packet in a subframe on HS-DPSCH, it must first receive an indication on HS-SCCH that the packet is intended for it as well as information needed for demodulating allocated code channels. That information is provided in the first of two parts making up an HS-SCCH subframe. The first part carries information for use in configuring physical level reception of relevant codes (allocated code channels and applied modulation). The second part provides HARQ information, needed for decoding a data transmission in case either the packet is in a first transmission or in a retransmission. The HARQ information in the second part makes it also possible to soft-combine the latest transmission with the already-received earlier transmission(s), before making new attempt of coding. Therefore, so that a terminal has what it needs in time to decode a packet intended for it, the control information is transmitted ahead of the packet, although the number assigned to the packet subframe (on HS-DSCH) is the same as the number assigned to the subframe conveying the control information (on HS-SCCH). Similarly, the L1 ACK/NACK signalling on HS-DPCCH for a packet is in a subframe assigned the same number as for the packet, but obviously occurs later in time, after the packet is received (either successfully, i.e. is decoded, or unsuccessfully).
It turns out that especially when a UE is in soft handover, it is more difficult for a Node B in communication with the UE to distinguish between ACK/NACK signalling by the UE over the HS-UP, i.e. when the UE is in a HARQ active state, and when the UE is not in a HARQ active state. When a UE is not in a HARQ active state, it enters what is called discontinuous transmission (DTX) mode. To avoid requiring that the UE transmit at a power level that for most time intervals would be unnecessarily high, the prior art proposes using preamble and postamble signalling to in effect more clearly enunciate to the Node B signalling associated with a HARQ active state and signalling associated with a DTX mode. (See e.g. 3GPP TSG-RAN WG1 Meeting #28, Washington, USA, 19-22 August 2002, Change Request R1-02-1085, entitled Correction of DTX transmission in ACK/NACK field.) According to the preamble and postamble signalling proposed by the prior art, when a UE receives signalling information directed to it on HS-SCCH, the UE would transmit a NACK in the subframe before the subframe allocated to the HARQ ACK/NACK. In addition, the UE would transmit a NACK in the subframe following the HARQ ACK/NACK, unless another HS-DSCH packet follows immediately and is successfully decoded. Such a procedure would keep the Node B from detecting DTX as ACK in the HARQ ACK/NACK subframe, allowing a substantial reduction in the required HS-UP power. Without such a change, the transmit power on the HS-DPCCH for ACK/NACK fields would have to be much higher than with the change. An accompanying technical document—R1-02-1085 entitled Reduction of HS-DPCCH power requirements (agenda item 7.4)—gives a detailed discussion.
The additional preamble/postamble signalling according to the prior art promises possible improved gain at the Node B (in determining whether the UE is in HARQ active mode or DTX mode) compared to the current signalling scheme, at least in case of random errors on HS-SCCH (which then causes the UE to enter DTX mode). However, in soft handover, it turns out that errors on HS-SCCH tend to be bursty and, it also turns out, the additional preamble/postamble signalling according to the prior art is less effective than in case of random errors. In addition, in recent prior art approaches, ‘HARQ inactivity mode,’ which in the earlier approaches was indicated as DTX transmission, is converted to look like NACK transmission, making inactivity appear similar to unsuccessful reception. While preamble/postamble signalling according to the prior art does likely improve gain in case of random errors, it should not be used when incremental redundancy (IR) is applied in HARQ (because, for one thing as explained above, the recent prior art preamble/postamble signalling makes it difficult to distinguish HARQ inactivity from unsuccessful decoding, which are both indicated by sending NACK), or in other words, IR and preamble/postamble signalling are mutually exclusive techniques to improve performance of HSDPA.
More importantly, there are sequences of ‘inactivity’ in which a UE still sends DTX (i.e. enters DTX mode), but the Node B expects a response of either ACK or NACK. For example, when three HS-SCCH control subframes are lost in sequence, a UE will send a sequence: NACK-DTX-NACK. The Node B might easily decode this as the sequence: NACK-ACK-NACK.
In an earlier suggestion, a long sequence of NACK is sent after relevant ACK/NACK symbols. Such a sequence cannot indicate for the Node B when a UE is in DTX mode, and, in addition, results in what is an often an unnecessary uplink transmission.
Thus, what is needed is a way by which a UE can signal to a Node B via HS-UP information the Node B can use to more readily determine when the UE is in DTX mode, especially during soft handover, where errors are bursty.